The present invention relates to power conversion apparatus and methods, and more particularly, to switching converters and methods of operation thereof.
DC-to-DC power conversion is commonly achieved using switching regulator circuits. These switching regulator circuits often include one or more switching elements that selectively couple a DC power source to a primary winding of a transformer such that an AC voltage is produced on a secondary winding of the transformer. This AC voltage may then be rectified and filtered to produce a DC output voltage. The DC output voltage may be regulated by controlling the switching frequency and/or duty cycle of the switching elements.
It is generally desirable that the switching element(s) of a switching regulator operate at high frequencies to allow for the use of small and lightweight magnetic components (e.g., lightweight transformers and inductors). An unfortunate result of such high frequency switching operations may be increased power dissipation due to resistance and capacitance associated with the switching element(s). Accordingly, it is generally desirable for the switching element(s) of a switching regulator to switch under minimized current and/or voltage conditions to reduce power dissipation.
Examples of power converters that may provide reduced or xe2x80x9czero currentxe2x80x9d switching (ZCS) are described in U.S. Pat. No. 4,415,959 to Vinciarelli; and in U.S. Pat. No. 4,823,249 to Garcia, II. Examples of power converters that may provide for reduced or xe2x80x9czero voltagexe2x80x9d switching (ZVS) are described in xe2x80x9cDesign Review: 500 W, 40 W/in3 Phase Shifted ZVT Power Converter,xe2x80x9d Topic 4, SEM-900 Power Supply Design Seminar Manual, Unitrode Integrated Circuit Corporation; xe2x80x9cOptimum ZVS Full-Bridge DC/DC Converter with PWM Phase Shift Control: Analysis, Design Considerations, and Experimental Results,xe2x80x9d by Balogh et al., APEC ""94 Proceedings, pp. 159-165 (1994); xe2x80x9cA Novel Soft-Switching Full Bridge DC/DC Converter: Analysis, Design Considerations, and Experimental Results At 1.5 kW, 100 kHz,xe2x80x9d by Redl et al., PESC ""90 Proceedings, pp. 162-172 (1990); xe2x80x9cDesigning a Phase Shifted Zero Voltage Transition (ZVT) Power Converter,xe2x80x9d Topic 3, SEM-900 Power Supply Design Seminar Manual, Unitrode Integrated Circuit Corporation; xe2x80x9cA Fixed Frequency ZVS High Power SMR Converter with Zero to Rated Load Variation Capability,xe2x80x9d by Moshopolous et al., INT-ELEC ""92 Proceedings, pp. 351-358 (1992); xe2x80x9cUC3879 Phase-Shift Resonant Controllerxe2x80x9d Data Sheet, Unitrode Integrated Circuit Corporation; xe2x80x9cThe New UC3879 Phase Shifted PWM Controller Simplifies the Design of Zero Voltage Transition Full Bridge Converters,xe2x80x9d Application Note U154, Unitrode Integrated Circuit Corporation.
According to embodiments of the invention, a power conversion apparatus, e.g., a DC-to-DC converter, includes a transformer having primary and secondary windings. A first switching circuit has an input port configured to be coupled across a DC power source and an output port coupled to the primary winding of the transformer. A switch control circuit is operatively associated with the first switching circuit and causes the first switching circuit to alternately apply first and second polarity voltages to the primary winding. A second switching circuit is operative to transfer energy to a load from the secondary winding via a first capacitor responsive to application of the first polarity voltage to the primary winding and to transfer energy to the load from the secondary winding via a second capacitor responsive to application of the second polarity voltage to the primary winding.
According to embodiments of the invention, the first switching circuit comprises at least one switch, and the switch control circuit constrains the at least one switch to operate when current in the at least one switch falls to a predetermined level, e.g., substantially near zero. The switch control circuit may operate the at least one switch responsive to a current through the transformer. Alternatively, the switch control circuit may estimate a time when current in the at least one switch will reach the predetermined level and may operate the at least one switch based on the estimated time.
According to other embodiments of the invention, the first switching circuit comprises first and second half bridges, and the switch control circuit controls a time delay between operations of the first and second half-bridges. The switch control circuit may control the time delay such that the first and second half-bridges operate under substantially zero current switching conditions. In some embodiments of the invention, the switch control circuit maintains a fixed time delay between operations of the first and second half-bridges. In other embodiments, the switch control circuit controls the time delay responsive to a sensed current through the transformer and/or responsive to an input voltage applied to the switching circuit.
According to other aspects of the invention, the first switching circuit comprises at least one switch, and the switch control circuit constrains the at least one switch to operate when voltage across the at least one switch falls to a predetermined level, for example, substantially near zero. The first switching circuit may comprise a first half-bridge including first and second switches and a second half-bridge including third and fourth switches. The switch control circuit, in transitioning the first switching circuit from a first state in which the second and third switches are closed and the first and fourth switches are open and a second state in which the first and third switches are closed and the second and fourth switches are open, may open the second switch before closing the first switch such that a voltage across the first switch is reduced, for example, to a voltage substantially near zero, before the first switch closes. The switch control circuit, in transitioning the first switching circuit from the second state to a third state in which the first and fourth switches are closed and the first and third switches are open, may also open the third switch before closing the fourth switch such that a voltage across the fourth switch is reduced, for example, to a voltage substantially near zero, before the fourth switch closes.
In other embodiments of the invention, a power conversion apparatus includes a transformer having primary and secondary windings. First and second half-bridges are configured to be coupled across a DC power source and coupled to respective first and second terminals of the primary winding of the transformer. An output circuit is coupled to the secondary winding of the transformer and includes first and second capacitors and at least one inductor configured to be coupled to a load. The output circuit is operative to transfer energy to the load from the secondary winding via the first capacitor responsive to application of a first polarity voltage to the primary winding of the transfer and to transfer energy to the load from the secondary winding via the second capacitor responsive to application of a second polarity voltage to the primary winding of the transformer. A switch control circuit, operatively associated with the first and second half-bridges, varies a frequency at which the first and second half-bridges operate responsive to an output voltage produced by the output circuit.
In some embodiments of the invention, the switch control circuit is operative to control a time delay between operations of the first and second half-bridges. For example, the switch control circuit may control the time delay such that the first and second half-bridges operate under substantially zero current switching conditions.
For example, the switch control circuit maintains a fixed time delay between operations of the first and second half-bridges, or the switch control circuit may vary the time delay responsive to a sensed current and/or to an input voltage applied to the first and second half-bridges.
In still other embodiments of the invention, a power conversion apparatus includes transformer having primary and secondary windings. A switching circuit is coupled to the primary winding of the transformer and configured to be coupled to a DC power source, and is operative to couple the DC power source to the primary winding of the transformer with a first polarity in a first state and to couple the DC power source to the primary winding of the transformer with a second polarity in a second state. An output circuit is coupled to the secondary winding of the transformer and includes first and second capacitors and at least one inductor configured to be coupled to a load. The output circuit is operative to transfer energy to the load from the secondary winding via the first capacitor responsive to the first state of the switching circuit and to transfer energy to the load from the secondary winding via the second capacitor responsive to the second state of the switching circuit. The switching circuit may vary a frequency at which the switching circuit alternates between the first and second states to control an output voltage applied to the load.
In other embodiments of the invention, the output circuit, when the switching circuit is in the first state, delivers current from the secondary winding to the first capacitor responsive to current in the secondary winding exceeding a first level and reduces current in the secondary winding responsive to discharge of the first capacitor. The output circuit, when the switching circuit is in the second state, also delivers current from the secondary winding to the second capacitor responsive to current in the secondary winding exceeding a second level and reduces current in the secondary winding responsive to discharge of the second capacitor. Respective capacitances of the first and second capacitors may be such that respective discharge currents produced from respective ones of the first and second capacitors in response to respective ones of the first and second states of the switching circuit are sufficient to cause the output circuit to block current flow in the secondary winding.
In still other embodiments, the output circuit, when the switching circuit is in the first state, delivers current from the secondary winding to the first capacitor responsive to current in the secondary winding exceeding a current demand of the load and then discharges current from the first capacitor through the at least one output inductor to supply current to the load. The output circuit, when the switching circuit is in the second state, also delivers current from the secondary winding to the second capacitor responsive to current in the secondary winding exceeding a current demand of the load and then discharges the second capacitor through the at least one output inductor to supply current to the load. Respective capacitances of the first and second capacitors may be such that respective peak discharge currents produced from respective ones of the first and second capacitors responsive to respective ones of the first and second states of the switching circuit are greater than or equal to a current delivered to the load via the at least one inductor.
In yet other embodiments of the invention, the apparatus further comprises a sensor that senses a current in at least one of the primary and secondary windings. The switching circuit is responsive to the sensor such that the first switching circuit short-circuits the primary winding when the sensed current meets a predetermined criterion. In other embodiments, a switch control circuit predicts a time at which current in at least one of the primary winding and the secondary winding meets a predetermined criterion and causes the switching circuit to short the primary winding based on the predicted time.
According to another aspect of the invention, the output circuit comprises a second switching circuit, for example, a diode bridge, that controls current flow between the secondary winding, the first and second capacitors and the at least one output inductor. The second switching circuit may control current flow between the secondary winding, the first and second capacitors and the at least one output inductor such that, when the first switching circuit is in the first state, the second switching circuit clamps the first capacitor when current in the secondary winding is less than a first level, conducts current from the secondary winding to the first capacitor responsive to current in the secondary winding exceeding a second level, and reduces current flow in the secondary winding responsive to discharge of the first capacitor. When the first switching circuit is in the second state, the second switching circuit may clamp the second capacitor when current in the secondary winding is less than a third level, conduct current from the secondary winding to the second capacitor when current in the secondary winding exceeds a fourth level, and reduce current flow in the secondary winding responsive to discharge of the second capacitor.
According to method embodiments of the invention, first and second polarity voltages are alternately applied to a primary winding of a transformer. Energy is transferred to a load from a secondary winding of the transformer via a first capacitor responsive to application of the first polarity voltage to the primary winding. Energy is transferred to the load from the secondary winding via a second capacitor responsive to application of the second polarity voltage to the primary winding. The step of alternately applying first and second polarity voltages to a primary winding may comprise operating a switching circuit coupled to a DC power source and to the primary winding such that at least one switch of the switching circuit is constrained to operate when current in the at least one switch falls to a predetermined level. The step of alternately applying first and second polarity voltages to a primary winding may also comprise operating the switching circuit that at least one switch of the switching circuit is constrained to operate when voltage across the at least one switch falls to a predetermined level.
According to other method embodiments, the step of alternately applying first and second polarity voltages to a primary winding comprises controlling a frequency at which first and second half-bridges of a switching circuit coupled to a DC power source and to the primary winding operate. The step of alternately applying first and second polarity voltage may also include controlling a time delay between operations of the first and second bridges, e.g., to achieve reduced or substantially zero current switching. For example, a fixed time delay may be maintained between operations of the first and second half-bridges, or the time delay may be controlled responsive to a sensed current through the transformer and/or an input voltage applied to the switching circuit.